Hearing thresholds under acute hypoxia and relationship to slowing in the auditory modality

BACKGROUND: Acute hypoxia slows reaction time to visual stimuli and it has been proposed that this slowing can be accounted for by a decrease in perceived brightness. Auditory slowing could be accounted for in an analogous manner if hypoxia decreases perceived loudness, as reflected by an increase in pure-tone thresholds. However, evidence concerning the effects of hypoxia on auditory thresholds is contradictory and we performed an experiment to clarify this issue. Pilot work suggested that thresholds were raised due to noise from our breathing apparatus used to deliver the low O2 mixtures and we eliminated this artifact by measuring thresholds while the subjects held their breath. METHODS: The six subjects breathed either air as a control through an oro-nasal mask or low O2 mixtures to maintain arterial blood oxygen saturation at 74% while thresholds between 500-4000 Hz were measured with an audiometer during breathholding. RESULTS: Hypoxia produced a statistically significant but practically insignificant decrease in thresholds of 1 dB across all frequencies tested. CONCLUSIONS: Our results are consistent with the view that audition is relatively insensitive to hypoxia and that the slowing observed with auditory stimuli cannot be accounted for by an increase in auditory thresholds. Some alternative hypotheses which could account for this slowing are proposed.